Abstract: In order to solve the problems of quantitative assessment of commutation failure risk degree and rapid identification of risk areas in multi-infeed HVDC systems, the key factors affecting commutation failure of multi-infeed HVDC systems are studied in this paper. Then, based on the improved calculation method of critical voltage of commutation failure in part (Ⅰ) and the idea of normalization, a quantitative evaluation index of commutation failure risk degree is proposed, which can be used to judge whether a commutation failure occurs in different DC subsystems by its positivity and negativity. Its amplitude can reflect the voltage drop from the current state to the commutation failure (negative amplitude) or the voltage rise required from the current state to the absence of commutation failure (positive amplitude). Finally, based on the quantitative evaluation index, a fast identification method of commutation failure risk area under different fault severity is proposed. Based on PSCAD/EMTDC, the dual-infeed and three-infeed HVDC system models are built to verify the effectiveness of the commutation failure risk degree assessment results and the rapid identification of commutation failure risk areas. The results show that the proposed index can be used to quantitatively evaluate the commutation failure risk degree of multi-infeed HVDC systems, and can accurately identify the commutation failure risk area of multi-infeed HVDC systems under different fault severity.